Priority is claimed to Patent Application No. 2001-73574 filed in Republic of Korea on Nov. 24, 2001, herein incorporated by reference.
1. Field of the Invention
The present invention relates to a micro-switching device actuated by low voltage, using an electrostatic attraction.
2. Description of the Related Art
In general, an RF switch is a kind of switch for turning a device on or off by using electrostatic attraction to bring a structure into contact with a signal line. In this case, a predetermined voltage is applied to the signal line so as to generate an electrostatic attraction required. Here, the voltage required is determined by the rigidity of a spring supporting a microstructure. Preferably, the spring has low rigidity so as to allow actuation by a low voltage.
When a microstructure constituting a micro device is in contact with a signal line or an electrode, they may, however, be adhered to each other. This problem may also occur when a voltage is applied to and then removed from an electrode. As a result, the microstructure is kept in contact with the signal line, thereby preventing the proper switching control of the micro device.
To solve this problem, the restoring capability of an actuated structure must be strengthened, to make the actuated structure return back to its original position. Thus, the structure has to be supported by a spring of high rigidity. However, as described above, the voltage applied to an electrode must be increased in order to use a spring of high rigidity. Nevertheless, a spring of high rigidity is often adopted in a micro switching device at the present time, so as to prevent the adhesion of a micro device to a signal line or an electrode. As a result, the necessary voltage is increased, and thus it is very difficult to make a micro switching device that can be actuated by a low voltage.
FIG. 1A is a perspective view of a conventional micro-switching device. The micro-switching device is supported by anchors 13, which are fixed onto a substrate, and springs 14 which are formed on the anchors 13, and includes a membrane 15 above the substrate, a lower electrode 11 corresponding to the membrane 15, and insulating layers 12. If a voltage is applied to the lower electrode 11, an electrostatic attraction is generated to actuate the springs 14. Then, the membrane 15 approaches the lower electrode 11 due to the electrostatic attraction, comes into contact with a signal line 16, and is then switched on.
FIGS. 1B and 1C are views for explaining defects of a conventional micro-switching device. Here, for convenience""s sake, the defects are diagrammatically viewed with regard to a general representation of a conventional micro-switching device. FIG. 1B is a view of a micro-switching device in which a membrane 15 is actuated by applying power to a lower electrode 11, and FIG. 1C is a view of the micro-switching device in which the membrane 15 is actuated and approaches closely to the lower electrode 11. More specifically, while the membrane 15 is not in contact with the lower structure of the lower electrode 11 and insulating layers 12, with its body held by the springs 14, an electrostatic attraction is generated between the membrane 15 and the lower electrode 11 when a voltage is applied to the lower electrode 11, thereby attracting the membrane 15 to the lower electrode 11. At this time, the more closely the membrane 15 approaches the lower electrode 11, the more the electrostatic attraction between the membrane 15 and the lower electrode 11 is increased. As a result, the displacement of the membrane 15 increases. Then, the displacement of the springs 14 increases to increase their restoring capability.
Here, the electrostatic attraction between the membrane 15 and the lower electrode 11 is calculated by the following equation:                               F          E                =                              1            2                    ⁢                                    ϵ              ⁢                              xe2x80x83                            ⁢                              AV                2                                                                    (                                                      g                    0                                    -                                      U                    z                                                  )                            2                                                          (        1        )            
wherein FE denotes an electrostatic attraction, A denotes a corresponding area, V denotes voltage applied to the lower electrode 11, Uz denotes the driving distance of the membrane 15, and g0 denotes a distance between the membrane 15 and the lower electrode 11. As shown in the equation (1), an increase in the driving distance Uz of the membrane 15 results in an increase in the electrostatic attraction FE.
The restorability capability of the springs 14 can be expressed by the following equation:
xe2x80x83Fs=kUzxe2x80x83xe2x80x83(2)
wherein Fs denotes the restoring capability of the springs 14, k denotes a spring constant, and Uz denotes the displacement of the membrane 15. From the equation 2, it is noted that the restoring capability Fs of the springs 14 increases linearly according to the displacement of the membrane 15.
FIG. 2 is a graph illustrating the relationship between the restoring capability of the springs 14 and the electrostatic attraction due to the displacement of the membrane 15. This graph reveals that the electrostatic attraction changes greatly, and the restoring capability of the springs 14 changes linearly, according to the driving distance of the membrane 15. The electrostatic attraction may be greater than or less than the restoring capability of the springs 14 according to the displacement of the membrane 15. This is caused by the use of a spring having a relatively large spring constant, or a low voltage applied to the lower electrode 11. Then, the driving distance of the membrane 15 is limited, i.e., it is actuated to a predetermined point and does not operate, and thus cannot function as a switch. However, referring to FIG. 2, the electrostatic attraction is always greater than the restoring capability of the springs 14, at which time the membrane 15 becomes in contact with the lower structure of the lower electrode 11, the insulating layer 12, and the signal line 16, due to the electrostatic attraction. At this time, the membrane can function as a switch.
Once a voltage is applied to the lower electrode 11, the membrane 15 comes into contact with the signal line 16, i.e. it is switched on, and thus the electrostatic attraction is far greater than the restoring capability of the springs 14. Then, the voltage is removed to make the membrane 15 switch off. However, adhesion, which is an inherent property of a micro device, may occur between the membrane 15 and the lower structure of the lower electrode 11, the insulating layer 12 and the signal line 16, thereby reducing the restoring capability of the springs 14. To prevent a reduction in the restoring capability of the springs 14, a spring having a large spring constant K may be used, but this is disadvantageous because a high voltage must be applied to the lower electrode 11.
The above problem can be solved by applying a predetermined force to the micro-switching device so that the membrane can return back to its original position without using a spring of high rigidity. That is, a spring of low rigidity is used, and means for applying a predetermined force onto the micro-switching device is additionally installed to separate the membrane from a lower structure.
For instance, electrodes for applying a driving force may be installed at the top as well as the bottom of the membrane. To actuate a microstructure and make it return back to its original position, a voltage is applied to the upper and lower electrodes of a microstructure. Then, the membrane may be driven in both directions, i.e. upward and downward, and thus can be easily separated from the electrodes to return to its original state. However, this method is disadvantageous in that the manufacturing process is complicated, thereby reducing the yield. Also, in fact, it is difficult to obtain sufficient restoring force to actuate the microstructure and return it to its original state with a low voltage.
To solve the above problems, it is an object of the present invention to provide a micro-switching device that can be actuated by a low voltage, easily deforms with a electrostatic attraction, and prevents the adhesion between elements while using a spring of low rigidity.
To achieve the object, there is provided a micro-switching device, including a spring operating elastically; a membrane formed on one side of the spring, being held by the spring; and a lower electrode formed below the membrane, for generating an electrostatic attraction when a voltage is applied thereto, wherein the membrane is non-planar.
Preferably, the spring is formed on an anchor which is formed on a substrate, and the membrane is actuated not to be in contact with the substrate while being held by the spring.
Preferably, the micro-switching device further includes a means for applying voltage to the membrane and the lower electrode.
Preferably, the lower surface of the membrane has a concave portion or protrusion, and the membrane is cut partially spherical.